As the galvanizing of steel sheet is the most effective and economical means for protecting it from corrosion by a galvanic action, ten millions of tons of crude steel, which correspond to about 10% of the yearly crude steel production of Japan (about 100 millions of tons), are used for the manufacture of galvanized steel sheets which are used in a wide variety of fields including their use as building materials, and as materials for automobiles and electric appliances. Zinc manifests a galvanic action as described below. The two metals, zinc and iron, contacting each other form a cell in which zinc, which is the baser metal, forms the anode, while iron becomes the cathode. Thus, it restrains any anodic dissolution by a local cell as formed by iron alone, and thereby prevents its corrosion. This rust-preventing action ends upon loss of all of zinc contacting iron, and in order to sustain the action for a long period of time, it is necessary to restrain the corrosion of the zinc layer, and it is, therefore, common practice to coat the zinc layer with chromate.
The chromate coating of galvanized steel sheet has, however, the drawback of having a black patina formed on the sheet during its storage or transport and impairing its appearance seriously, though it may drastically improve its corrosion resistance (anti-white rust property) It is known that a black patina is likely to appear on, among others, galvanized steel sheet subjected to skin pass after galvanizing, or produced in a galvanizing bath containing lead, or coated with a zinc layer containing several percent of aluminum.
The black patina is characterized by a grayish black color presented by spangles formed in a galvanized surface and having a specific crystalline orientation, and it is, therefore, effective to minimize the spangles by a known method to restrain the appearance of a black patina to some extent. It is also known that the use of a galvanizing bath containing only a very small amount of lead (Pb not exceeding 0.01% by weight) makes a black patina less likely to appear, since the spangles in which a black patina appears contain lead particles forming the active sites which promote the appearance of a black patina. The addition of lead to a galvanizing bath is, however, unavoidable, since many users of galvanized steel sheets like spangles.
According to pages 939 to 946 of TETSU & HAGANE, Vol. 77 (1991), published by The Japan Iron and Steel Association, the spangles are classified into seven types, i.e. fern I, fern II, mirror, frost, half fern, feather, and triangle types, and the frost type spangles are particularly likely to be enriched with Pb and Al. The users who like spangles generally prefer a surface having many frost type spangles as presenting a beautiful appearance, but a black patina is particularly likely to appear in frost type spangles, since they are enriched with Pb and Al, as stated above.
Japanese Patent Application Laid-Open No. Sho 59-177381 proposed flashing treatment with an aqueous solution containing Ni or Co ions (for the chemical deposition of a very small amount of metal) as a method of preventing a black patina from appearing after chromate treatment, and the flashing treatment has recently come to be considered as an effective method of preventing a black patina from appearing after chromate treatment. According to the disclosure of Japanese Patent Application Laid-Open No. Sho 59-177381, the surface of a zinc or zinc-alloy plated steel sheet is given flashing treatment with an aqueous solution having a pH of 1 to 4, or 11 to 13, and containing Ni or Co ions, or both prior to its chromate treatment, whereby the metal ions are deposited in metallic or oxide form on the sheet surface, and after it is washed with water, a chromate film is formed thereon.
There is not yet any definite opinion about the mechanism which enables the flashing treatment of a zinc or zinc-alloy plated steel sheet with Ni or Co to prevent a black patina from appearing thereon after its chromate treatment, but according to the statement on pages 150 and 151 of the Preprint for the 60th Scientific Lecture Meeting of the Association of Metal Surface Technology, the metal as deposited by flashing is mostly found in the grain boundary of zinc crystals, and the chromium compound as deposited by the subsequent chromate coating treatment is likewise distributed in the grain boundary, and it can, therefore, be presumed that some interaction occurs between the metal as deposited by flashing and the chromium compound, and causes the latter to be adsorbed and fixed to the former.
The black patina formed on a zinc or zinc-alloy plated steel sheet looks black, apparently because the basic zinc carbonate of which it is composed, and which is represented as (ZnCO.sub.3)x.[Zn(OH).sub.2 ]y, like white rust, has a particle diameter falling within the visible light wavelength range of 400 to 700 nm, and is, therefore, very likely to scatter and absorb light. The black patina is considered as a product of corrosion formed in an environment lacking oxygen, and particularly with the progress of corrosion from the grain boundary. Accordingly, it is considered that the chromium compound with which the grain boundary is enriched by the metal deposited by flashing restrains the corrosion from the grain boundary and thereby contributes to preventing the formation of a black patina. Thus, the flashing treatment of a zinc or zinc-alloy plated steel sheet with Ni, Co, etc. prior to its chromate treatment can be an effective means for preventing the formation of any black patina thereon.
It has, however, been found that the flashing treatment carried out prior to chromate treatment makes white rust more likely to form, though it may restrain the formation of a black patina. This is apparently due to the fact that Ni or Co as deposited on the galvanized surface by the flashing treatment forms a local cell with zinc.
It is, therefore, an object of this invention to provide a process which can manufacture a chromate-coated lead-containing galvanized steel sheet having excellent anti-black patina and anti-white rust properties without relying upon any flashing treatment with Ni, Co, etc., and more particularly, a chromate-coated lead-containing galvanized steel sheet which hardly has any black patina formed thereon, even if it may have many spangles of the frost type which is liked by many users.